Techniques have been recently developed for the processing of cellulosic-based fabrics using substantially anhydrous liquid ammonia as a processing liquid. One of the important early developments in this connection is reflected in the Lindberg et al U.S. Pat. No. 3,406,006. Important improvements on the basic Lindberg et al process are reflected in the Walter S. Troope et al U.S. application Ser. No. 577,613, filed May 14, 1975, now U.S. Pat. No. 3,980,429. In the liquid ammonia process, as described in the Lindberg et al and Troope et al patents, a relatively dry fabric, having a substantial content of cellulosic fiber, is immersed in or otherwise impregnated with substantially anhydrous liquid ammonia, typically at a temperature of around -33.degree. C. Within a few seconds after impregnation, the liquid ammonia is driven out of the fabric with heat, in order to terminate the operative reactions between the cellulosic fiber and the ammonia. To advantage, this is accomplished by means such as a so-called Palmer dryer, with one or more drying stages as may be appropriate.
Experience has indicated that only about 93-95% of the ammonia content of the fabric after impregnation is removable through the use of dry heat, at least within a time frame appropriate to a continuous commercial process. The residual ammonia is weakly bonded to the cellulosic material of the fabric. Because of the extremely high affinity of water to the cellulose, the more weakly bonded residual ammonia may be removed by contacting of the fabric by water. However, a water spray or water dip operation would require a drying step, and would be both costly and impractical. Heretofore, efforts have been made, with partial success, to remove residual ammonia by subjecting the fabric to the action of high pressure jets of steam. However, exposing the fabric to steam jets is not entirely effective, because the steam in itself is of a "dry" character. In this respect, if the steam used for this purpose is not superheated, it will condense upon the equipment, causing staining and other problems.
In practice, utilizing a combination of dry heat from the Palmer stage, and steam purging and holding time in a subsequent processing chamber, it has been practical in a continuous commercial process to reduce the residual ammonia to about 1% by weight of the fabric. While, in time, this residual ammonia content will be released and evaporated, it can present a problem with regard to the working environment of the process operators. Thus, even where substantial ventilation is provided, a residual ammonia content of around 0.1% is a desirable maximum. Ammonia has a very strong characteristic odor, which can be disagreeable to some individuals and is detectable in the air in concentrations of as low as five parts per million.
In accordance with the invention, significant further reductions in residual ammonia content are made possible by effecting a surface contacting of the fabric with a very thin film of water, adequate to achieve a highly effective release of the weakly bonded ammonia while at the same time insufficient to require a subsequent drying stage. Most advantageously, contacting of the fabric by a film of the character described is achieved by passing the fabric through a secondary treatment zone in which, in addition to being exposed to the action of steam jets, the fabric is passed successively over a pair of chill rollers, contacting first one surface of the fabric and then the other. The chill rollers are supplied with a cooling medium such that there is a continual surface condensation formed on the chill rollers, by condensation of the dry steam atmosphere which prevails in the chamber. It has been found that this light film moisture, brought into contact with opposite surfaces of the fabric, effectively releases the weakly bonded ammonia from the fabric. Thereafter, the fabric can be passed over a heated roller, which serves to drive the moisture into the fabric. Eventually this moisture is evaporated, together with the combined ammonia before the fabric emerges from the processing chamber. By condensing the steam onto the chill rollers, a precise control over the moisture film is possible, enabling the fabric to be surface moistened without becoming fully wetted.
After surface contacting of the fabric with a water film and subsequent passing of the fabric over a heated roller, the fabric may again be exposed to one or more dry steam purging operations in yet another processing zone. This removes some of the moisture imparted by the chill rolls and along with it most of the residual ammonia, which is displaced by the moisture because of the greater affinity of the water for the fabric than of the ammonia for the fabric. The fabric ultimately discharged from the processing chamber has an extremely low residual ammonia content suitable in most instances to satisfy the environmental requirements and other considerations.